Abstract: The present paper deals with lithology and various sedimentary structures in the lower and middle members of the Lower Triassic Xikou Formation, Fujian. The authors contend in this paper that the early Indosinian environments in southwestern Fujian should be a lower slope and base of slope environment in a deep-sea basin with the following reasons. (1) lithologically, higher porpotions of finer and thinner distal turbidites occur in the lower member of the Xikou Formation, and Locally proximal turbidites in the middle member of it; (2) The structural characteristics of some of graded units composed of sand-silt-mud admixtures are similar to those of the immature "dirty sand" or graywacke in the ancient flysch formations; (3) The sedimentary structures such as scour marks, tool marks, flute casts, groove casts, flame and load structures, convolute bedding, ripple drift microlamination show that the activity of turbidity current and fluidized flow are strong; (4) Compared with the Late Permian, there are considerable variations in the Indosinian faunas in Fujian. The lower abundance and diversity of these faunas suggest that the environments of the Xikou Formation may be a deep sea characterized by planktonic bivalves.
Tectonically,it is obvious that a deep-sea basin created by tensional deep fracture once occurred in the central part of southwestern Fujian depression, and the transgression had its largest scale, although the depth and scale of the basin is still a puzzle.